(1) Field of the Invention
The present invention relates to titration methods, and more specifically to a potentiometric titration method for a quantitative determination of hydrogen peroxide.
(2) Description of the Prior Art
There continues to be a need for energy sources with a high energy density. In particular, there is a need for high energy density energy sources that can power unmanned undersea vehicles. Such energy sources when used to power such vehicles are required to have an energy density greater than 600 Wh kg−1. They also need to have long endurance and quiet operation. Additionally, they must be relatively inexpensive, environmentally friendly, safe to operate, reusable, capable of a long shelf life and not prone to spontaneous chemical or electrochemical discharge.
The zinc silver oxide (Zn/AgO) electrochemical couple has served as a benchmark energy source for undersea applications. Because of its low energy density, however, it is not suitable for unmanned undersea vehicles whose energy density requirements are seven times those of the Zn/AgO electrochemical couple.
In an effort to fabricate power sources for unmanned undersea vehicle with increased energy density (over zinc-based power sources), research has been directed towards semi fuel cells (as one of several high energy density power sources being considered). Semi fuel cells normally consist of a metal anode, such as magnesium (Mg) and a catholyte such as hydrogen peroxide (H2O2). In general the performance and health of these types of semi fuel cells are a function of the quantity of hydrogen peroxide in the catholyte. The key to achieving a high energy density for these types of semi fuel cells lies in the efficient usage of the hydrogen peroxide. The electrochemical processes during cell discharge are:Anode: Mg->Mg2++2e−  (1)Cathode: H2O2+2H++2e−->2H2O  (2)The voltage at the cathode and the total semi fuel cell voltage are directly related to the concentration of hydrogen peroxide in the catholyte according to the Nernst equation:E=E0+(0.0591*log([H2O2]*[H+]2))/2  (3)where E is the half cell voltage at the cathode, E0 is the standard voltage at unit activity of H2O2 and H+, and [H2O2] and [H+] are the molar concentrations of peroxide and protons respectively. Equation (3) shows that as the peroxide concentration decreases so does the cell voltage.
It is important to directly monitor and control the hydrogen peroxide concentration [H2O2], because the concentration is used to assess the functional condition and performance of the semi-fuel cell. For example, if the hydrogen peroxide concentration differs significantly from expected levels for a given semi fuel cell load, then the pump controlling the hydrogen peroxide input can be directed to increase or decrease the amount of hydrogen peroxide being pumped into the semi fuel cell.
In a laboratory environment, measurement of hydrogen peroxide concentration in a semi fuel cell is performed using a colorimetric titration method. In this method, a solution of unknown peroxide concentration is colored with a small amount of indicator material such as iron(II) 1,10 phenanthroline. Then, a chemical of known concentration, typically cerium (IV) in sulfuric acid solution, (the titrant solution) is added that reacts with peroxide. When the solution turns clear, all of the hydrogen peroxide has been consumed. There is a 2:1 correlation between the number of titrant reactant molecules consumed during the titration and the number of hydrogen peroxide molecules initially present in the solution when cerium (IV) is used. The concentration of hydrogen peroxide can be determined using this correlation. This method is not suitable for use in an unmanned undersea vehicle, however, because it requires visible detection of a color change by a human operator. Currently there is no automated means for quantifying the concentration of hydrogen peroxide in a semi fuel cell onboard an unmanned undersea vehicle.
What is needed is a method of quantifying the concentration of hydrogen peroxide in a semi fuel cell catholyte that is automated and can provide concentration data that can be interpreted by a digital processor.